1. Field of the Invention
The present invention provides an electrophotographic photoreceptor. More particularly, the present invention provides a high-performance electrophotographic photoreceptor having excellent light and ozone resistance.
2. Discussion of the Background
Owing to its instantaneous nature and ability to provide high-quality images, electrophotography is used extensively not only in copiers, which are a conventional application, but also in various printers, facsimile telegraphs, and the like.
Inorganic photoconductive materials, such as amorphous silicon and arsenic-selenium systems, are presently employed as part of the photoreceptors, which are the nucleus of electrophotography. However, use of organic photoreceptors remains in the majority.
Multi-layer arrangements have been developed for organic photoreceptors. However, an arrangement in which the multi-layered photosensitive layer consists of a charge generation layer and a charge transport layer, so that the function of charge generation and that of charge transport are separately allotted, is currently being enthusiastically investigated/developed. This is because this type of photosensitive layer has a high degree of freedom of design and, hence, would enable a higher-performance photoreceptor to be produced, as well as having a high productivity, etc. At present, the range of uses thereof has spread even to medium- to high-speed copiers and printers.
The properties required of photoreceptors include the following basic properties: to have high photosensitivity; to have sufficient charge acceptance capacity; to be reduced in dark decay after mechanical light irradiation; to show a low residual potential; to show satisfactory response characteristics; and to be highly stable in these properties in repetitions of use. In addition to the foregoing, various properties are required from the standpoint of practical use.
One of these is ambient light resistance. Usually, the photoreceptor mounted in a copier or laser printer is used in the state of being shielded from ambient light. However, during machine assembly or when the photoreceptor is taken out of the machine due to, e.g., paper sticking, the photoreceptor is inevitably exposed to the ambient light. This ambient light has a far higher intensity than the exposure/mechanical light to be used for image formation in the machine and, hence, causes considerable damage to the photoreceptor. This is because a large amount of charge traps generate within the photoreceptor when the photoreceptor is exposed to light, and in many cases this leads to a considerable increase in residual potential.
Although the mechanism by which charge traps generate has not been fully elucidated, the following hypothesis exists. For example, the charge transport material itself absorbs the exposure/mechanical light and is thus excited. When this charge transport material relaxes from the excited state, it does not return to the original ground state but changes into another structure having an intermediate energy state, and this is causative of the charge traps. In another case, an ingredient in the charge transport layer (e.g., the charge transport material alone or, in the case where substance having an electron affinity is contained, a weak charge-transfer complex formed from the substance having an electron affinity and the charge transport material) is directly excited to generate charge carrier pairs, which result in the charge traps.
On the other hand, various charging techniques are employed in copiers and laser printers. It is known that around the high-voltage charging units, oxygen molecules contained in the air are ionized to generate ozone. It is also known that the ozone thus generated causes damage to the photoreceptor. Although the mechanism of this phenomenon also has not been fully elucidated, it is thought that the deterioration of the photosensitive material by ozone, which is an oxidizing substance, and the resultant charge traps are causative.
Heretofore, techniques used to prevent damage to the photoreceptor include the following. For diminishing the influence of ambient light, use has been made, for example, of a method in which a yellow lamp, which is less influential, is employed as an illuminator in machine assembly and a method in which when the machine is opened, a light-shielding plate is disposed in order to minimize the influence of exposure to ambient light on the photoreceptor. For diminishing the influence of ozone, a method in which a contact type charging device, which is less apt to generate ozone, has been employed, as well as a method in which a fan is employed to discharge the ozone generated outside the apparatus.
On the other hand, materials thought to be unsusceptible to oxidation are being investigated for use, such as the incorporation of an electron-attracting substance or an antioxidant into a charge transport layer as described in, e.g., JP-A-7-191476 and JP-A-5-323631. However, these techniques have been insufficient for preventing an increase in residual potential and the effect of inhibiting a decrease in electrification characteristics.
In particular, there have been problems, for example, that use of an antioxidant alone is ineffective in inhibiting the influence of exposure to ambient light and produces higher side effects on other electrophotographic properties. As such, there remains a critical need for high-performance electrophotographic photoreceptors that have excellent light and ozone resistance.